Magnetic knot games: Physicists have demonstrated for the first time that the exotic Hopfion rings can be divided in a controlled manner – this is important for future spintronic computers and data storage. Hopf ions are magnetic 3D structures in materials that are created by a specific arrangement of atomic spins. Similar to knots, they are considered topological structures whose basic characteristics remain intact even when deformed. But in the experiment, the researchers were able to overcome this.
Under the influence of magnets, some materials can form exotic spin structures – geometric structures formed from the atomic spins of the particles with special, particle-like features. These include vortex-like skyrmions, but also the hopfions, which were experimentally proven for the first time just a few years ago. These are ring-shaped 3D structures that are considered a magnetic counterpart to classic knots.
The highlight: Such nodes are topological objects. This means that their basic characteristics, such as the number of loops and thread crossings, cannot be changed simply by deformation. Because of their stability, such magnetic node structures are considered promising carriers of quantum physical information, for example in quantum computers or new types of magnetic storage. To do this, however, it is necessary to actively control the topology of the Hopf ions, which is expressed by the so-called Hopf number.
Turn one into two or four
At this point, physicists have now made important progress. A team led by Shoya Kasai from the University of Tokyo has discovered for the first time how hop ions can be divided. A Hopfion ring with a high Hopf number creates several new Hopfions with lower Hopf numbers. Which Hopf ions are created follows a simple logic: the Hopf number must remain the same before and after this division. A four-fold hopfion can therefore be divided into two two-rings or four one-hopfions.
“This finding paves the way to hierarchical and dynamic control of node topology that is broadly applicable in several disciplines,” the physicists state. The ring-shaped spin structures form relatively interference-resistant magnetic storage units that can be manipulated in a targeted manner despite their stability.
Spin current as a control element
The controlled splitting of the Hopfions is possible using so-called spin orbit torques (SOT). They are formed when a layer of a ferromagnetic material is combined with an underlying layer of a heavy metal. If a voltage is applied to the heavy metal, it generates a spin current that runs perpendicular to the metal layer. In this case, electrons with the same spin flow in one direction, while electrons with exactly the opposite spin flow in the other direction. Unlike electric current, no charges are transported.
When the spin current enters the magnetic material, it acts on the Hopf ions present there and pulls these rings apart, as the physicists explain. If the spin current exceeds a certain intensity threshold, the Hopfion rings tear up. “The spin current causes the Hopf ions to split, leaving behind several Hopf ions with lower Hopf numbers,” write Kasai and his colleagues.
First step towards spintronic computers?
“This controllability of the node topology is a unique property of the Hopf ions, which means they can be stabilized with any Hopf numbers,” say the physicists. “Our results pave the way for Hopfion-based multistage storage devices that exploit the topological degrees of freedom of nodes and their spin current-controlled convertibility.” However, there are still a few challenges to be overcome before the first computer or data storage device that works with Hopfions is created.
Source: Shoya Kasai (University of Tokyo) et al., APS Open Science, 20ß26; doi: 10.1103/ngvg-h27j